Acoustic wave sensors are utilized in a variety of sensing applications, such as, for example, temperature and/or pressure sensing devices and systems. Acoustic wave devices have been in commercial use for over sixty years. Although the telecommunications industry is the largest user of acoustic wave devices, they are also used for sensor applications, such as in chemical vapor detection. Acoustic wave sensors are so named because they use a mechanical, or acoustic, wave as the sensing mechanism. As the acoustic wave propagates through or on the surface of the material, any changes to the characteristics of the propagation path affect the velocity, phase and/or amplitude of the wave.
Changes in acoustic wave characteristics can be monitored by measuring the frequency or phase characteristics of the sensor and can then be correlated to the corresponding physical quantity or chemical quantity that is being measured. Virtually all acoustic wave devices and sensors utilize a piezoelectric crystal to generate the acoustic wave. Three mechanisms can contribute to acoustic wave sensor response, i.e., mass-loading, visco-elastic and acousto-electric effect. The mass-loading of chemicals alters the frequency, amplitude, and phase and Q value of such sensors. Most acoustic wave chemical detection sensors, for example, rely on the mass sensitivity of the sensor in conjunction with a chemically selective coating that absorbs the vapors of interest resulting in an increased mass loading of the SAW sensor. Examples of acoustic wave sensors include acoustic wave detection devices, which are utilized to detect the presence of substances, such as chemicals, or environmental conditions such as temperature and pressure.
An acoustical or acoustic wave (e.g., SAW/BAW) device acting as a sensor can provide a highly sensitive detection mechanism due to the high sensitivity to surface loading and the low noise, which results from their intrinsic high Q factor. Surface acoustic wave (SAW/SH-SAW) and amplitude plate mode (APM/SH-APM) devices are typically fabricated using photolithographic techniques with comb-like interdigital transducers (IDTs) placed on a piezoelectric material. Surface acoustic wave devices may have a delay line, a filter or a resonator configuration. Bulk acoustic wave devices are typically fabricated using a vacuum plater, such as those made by CHA, Transat or Saunder. The choice of the electrode materials and the thickness of the electrode are controlled by filament temperature and total heating time. The size and shape of electrodes are defined by proper use of mask. Based on the foregoing, it can be appreciated that acoustic wave devices, such as a surface acoustic wave resonator (SAW-R), surface acoustic wave filter (SAW-filter), surface acoustic wave delay line (SAW-DL), surface transverse wave (STW), bulk acoustic wave (BAW), can be utilized in various sensing measurement applications.
One promising application for micro-sensors involves oil filter and oil quality monitoring. Except under very unusual circumstances, oil does not “wear out”, “break down” or otherwise deteriorate to such an extent that it needs to be replaced. What happens is that it becomes contaminated with water, acids, burnt and un-burnt fuel, carbon particles and sludge so that is can no longer provide the desired degree of protection for engine components. Most oil filters in modern vehicles do not remove all the contaminants. A filter can only remove solid particles above a certain size. It cannot remove water, acids, or fuel dilution, all of which pass through the full-flow filter just as readily as the oil.
Motor oils are fortified with inhibitors to provide them with a remarkable stability and resistance to oxidation and deterioration. Such oils also contain acid neutralizing additives to eliminate acidity or engine corrosion. There is a limit, however, to the amount of contamination that even the best oil can neutralize, and there comes a time when the only satisfactory procedure is to drain the oil and replenish the engine with a new charge. Thus, there arises the necessity for regular oil changes.
The question is now “how often should engine oil be changed?” Unfortunately, there is no simple answer to this question. From the foregoing discussion, it is apparent that oil is changed not because it has deteriorated, but because it has become contaminated with various harmful substances, and that the greater the rate at which such substances enter the oil, the sooner an oil change is necessary.
Factors that influence the necessity of oil changes include engine conditions and the method of engine operation. A vehicle that is used mainly for short distance stop-start running will require more frequent oil changes than one used for regular long distance traveling. A warm engine with leaky piston rings, for example, can contaminate the oil quicker than a new engine in good mechanical condition.
It should also be kept in mind that a high performance product (e.g., more additives) can handle more contaminates than other products, and hence longer oil change periods can be justified. As a final comment on this subject, it is worth mentioning that it is normal for oil to darken in service. This is not an indication that the oil has deteriorated. This merely demonstrates that the oil has picked up contaminates and maintains them in suspension, where they can do no harm, and where they can be removed from the engine when the oil is changed.
In general, motor oil should perform two primary functions. The oil must lubricate the engine and also serve as a collector of contamination. The contamination comes from the engine combustion chambers where the gasoline is burned to produce powder. There are two different types of fuel combustion in engines: efficient combustion or clean burning; and inefficient combustion or dirty burning.
When dirty combustion occurs in an engine, soot is not the only product formed. Sticky, gummy products, which oil chemists refer to as resins, and lead oxyhalides, may also form. Small quantities of acidic combustion products may also be present. Water is also a factor. For every gallon of gasoline burned, a little over one gallon of water may be formed. Thus, during the burning of gasoline in engines, a potential problem exists with respect to soot, resins, acids, and water formation. If combustion products function past the pistons and manage to penetrate the crankcase oil, then a problem of dirty, contaminated oil will exist. If the oil is allowed to become too dirty and contaminated, sludge deposits can form, thereby resulting in plugged piston rings, oil pump screens and oil filters. Engine wear and even engine damage can then result.
A truck, bus or passenger car driven at highway speed on a long trip can easily be lubricated and is the least demanding on an oil of good quality. The really tough lubricating job is the engine, which typically experiences only short runs with numerous stops and starts, especially in cold weather. The worst conditions for both the engine and the oil are the very conditions under which the great majority of passenger cars are used most of the time.
Knowledge of the condition of oil in the field would obviously be extremely beneficial information to truck fleet maintenance managers and maintenance personnel. A permanently installed oil quality sensor system can deliver the above information.
Currently, fleets that do perform analysis on their lubes utilize complete laboratory oil analysis. Primarily due to the cost of laboratory analysis, however, these tests are only performed on a routine basis, i.e. monthly or at each oil drain interval. Laboratory oil analysis serves two basic functions. The first function is to monitor the condition of the lube oil. Lube oil within a healthy engine degrades at a slow rate with normal use. Therefore, lab analysis can provide a forewarning and allow for scheduling of routine oil drains. Complete lab analysis is very effective in accomplishing this goal and first function.
It is at the second function, however, where lab analysis fails and does not provide sufficient failure warnings such as coolant leaks and stress related metal failures. Equipment is normally sampled on a monthly basis and while this is a sufficient interval to safely monitor the lube condition, many times this frequency is not sufficient in detecting engine problems. After all, analysis is used to detect the “Problem” before “Failure” and “Downtime” can then occur.
An example of this situation is as follows. A company samples its equipment on a monthly basis. On the first day of the month a sample of the used oil is taken and sent to the lab for analysis. On the second day, unknown to the maintenance personnel and the oil lab, a coolant leak develops within the engine. The next scheduled time for another complete laboratory analysis sample to be taken is twenty-nine days away.
Within the next several days, the coolant leak degrades the oil within the engine to the point that it causes wear to occur to bearings and other parts of the engine. Somewhere between the seventh and the tenth day the operator receives the results from the lab sample taken on the first day of the month. These results were taken before the problem occurred and shows no problems within the engine and that the oil is suitable for further use. Two days after receiving this report, the operator notices that the oil is becoming cloudy and that the engine is making a little steam. The routine monthly sampling of the used oil was not effective in achieving its goal.
The need is immense for a permanently installed sensor device that can determine the condition of the lube and equipment which can be used on a more frequent basis than complete laboratory analysis sampling. This need can be met by the use of the disclosure here.
One promising application for micro-sensors involves oil filter and oil quality monitoring. Diesel engines are particularly hard on oil because of oxidation from acidic combustion. As the oil wears, it oxidizes and undergoes a slow build-up of total acids number (TAN). A pH sensor is capable of direct measurement of TAN and an indirect measurement of total base number (TBN), providing an early warning of oil degradation due to oxidation and excess of water. The acids and water build-up is also related to the viscosity of the oil.
Low temperature start-ability, fuel economy, thinning or thickening effects at high and/or low temperatures, along with lubricity and oil film thickness in running automotive engines are all dependent upon viscosity. Frequency changes in viscosity have been utilized in conventional oil detection systems. The frequency changes caused by small changes in viscosity of highly viscous liquids, however, are very small. Because of the highly viscous loading, the signal from a sensor oscillator is very “noisy” and the accuracy of such measurement systems is very poor. Moreover, such oscillators may cease oscillation due to the loss of the inductive properties of the resonator.
TAN is a property typically associated with industrial oils. It is defined as the amount of acid and acid-like material in the oil. Oxidation and nitration resins make up the majority of this material. As the oil is used, acidic components build up in the lubricant causing the TAN number to increase. A high TAN number represents the potential for accelerated rust, corrosion and oxidation and is a signal that the oil should be replaced. Critical TAN numbers are dependant on oil type.
There is a need to provide a sensor apparatus which can be utilized to monitor, in a sensitive manner, the etching effects of etchants, such as acids contained in oils. There is also a need to provide a sensor system which can monitor corrosion or degradation of engines or other devices caused by exposure to such etchants. It is believed that acoustic wave sensors may well be suited for such monitoring as indicated by the embodiments described herein.
One of the problems with acoustic wave devices utilized in oil monitoring applications, for example, is that frequency changes caused by small changes in the viscosity of highly viscous fluids, are very small. Because of highly viscous loading, the signal from an oscillator associated with the acoustic wave sensor device is very noisy and the accuracy of such measurements is very poor. Moreover, the oscillators may cease oscillation due to the loss of the inductive properties of the resonator.